The invention relates to coupled transmission line sensors or acoustic cable sensors, and more particularly to a coaxial cable with a sense conductor moveable relative to another conductor in response to intruder-caused movement or vibration and a system operative to process a signal coupled to the sense conductor to detect and locate characteristic impedance changes of the sense conductor due to the intruder-caused movement or vibration.
There are numerous acoustic cable outdoor perimeter security sensors on the market today. These are based on six main sensor technologies, including 1) electret effect, 2) inductive coupling, 3) capacitive coupling, 4) triboelectric effect, 5) piezoelectric effect, and 6) fiber optic transmission.
All of the foregoing technologies have been used to create line sensors, referred to as "sensor cables" "transducer cables", or "acoustic cables", which act as distributed microphones. Typical transducer cable "zone lengths" are from 10 to 300 meters. In many cases transducer cables have been attached to chain link fences to detect intruders climbing the fences or cutting through them. While some of the prior transducer cables are relatively low cost devices, they result in an excessive number of false alarms due to (1) rain or hail striking the cable, (2) wind blown objects hitting the fence, or (3) the wind induced motion of the fence itself.
In other cases, the known transducer cables are buried in the ground to detect seismic activity caused by intruders moving over the cable. The inability of detection systems using such transducer cables to accurately distinguish between intruders walking over the cable and vehicular traffic moving at a distance from the cable is a major cause of false alarms. When the number of false alarms is too high, the monitoring service or response force often merely turns the equipment off.
In every case, the transducer cable installer sets a single threshold which must be exceeded to cause an alarm. The setting of this threshold always is a compromise. If it is set too low, the number of false alarms is too large. If it is set too high, the probability of detecting an intruder is too low. The longer the length of the transducer cable, the more difficult the compromise becomes. This is because the longer the transducer cable, the more background noise it picks up, thereby decreasing the signal-to-noise ratio. Also, the longer the transducer cable, the larger is the variation in sensitivity of the cable to physical vibrations along its length of the cable. For fence applications, this variation in sensitivity can be due to variations in cable construction, variations in fence conditions, variations in installation, and attenuation in the sensor cable itself. For buried applications, sensitivity to physical vibrations is affected by imperfections in cable construction and changes in the properties of the ground or burial medium.
The above mentioned prior art electret sensor cables were introduced in the 1970's, and presently are probably the most commonly used acoustic sensor cable. An electret cable sensor includes a coaxial cable with an electret dielectric, such as Teflon. A permanent charge is imposed upon the Teflon during the cable fabrication. In some cases, manufacturers simply rely on the charge imposed on the cable during the manufacturing process, while in other cases the charge is deliberately imposed on the cable after it is manufactured by heating the cable to near its melting point and applying a voltage to the cable. This charge will remain in the cable dielectric for many years. When the cable is subjected to physical vibration, the relative motion of the cable conductors and the charge on the electret dielectric causes a corresponding voltage to be generated at the end of the cable. It is this voltage which is sensed to detect the presence of an intruder. The cost of the electret dielectric material is a significant factor in the overall cost of this type of sensor cable, as teflon is several times more expensive than polyethylene, which is the most common dielectric used in the manufacture of coaxial cables. An example of a commercially available electret sensor cable is the FPS-2 device made by Perimeter Products Inc. of Mountain View, Calif. U.S. Pat. Nos. 3,384,887 issued May 21, 1968, 3,763,482 issued Oct. 2, 1973 and 4,023,155 issued May 10, 1977 describe this technology.
The known capacitive coupling transducer cables are used by applying a voltage across the conductors of a coaxial cable using a very high impedance source and then detecting minute changes in current therein needed to maintain this voltage while flexing of the cable causes changes in its capacitance. Problems relating to the high impedance sources required for such sensor cables have limited their application. An example of a commercially available capacitive coupling sensor cable is a buried sensor cable made by H.E.S.A. of Milan, Italy, based upon U.S. Pat. No. 5,068,642 which issued Nov. 26, 1991.
Known inductive coupling transducer cables utilize permanent magnetic material with embedded conductors. The conductors are allowed to move within a slot in the magnetic material in response to acoustic stimulus, thereby generating a voltage at the end of the sense cable. The cost of the magnetic material and the difficulty in manufacturing cable using the magnetic material are the most significant factors in determining the cost of inductive coupling transducer tables. An example of a commercially available inductive coupling transducer cable is the GUARDWIRE device produced by Geoquip Corp. of Wirksworth, United Kingdom and sold in the United States by Southwest Microwave Inc. of Tempe, Ariz.
Known triboelectric transducer cables are constructed using special plastic materials that generate a voltage when one moves against the other. Coaxial cables made with these materials, when flexed, generate a voltage at their terminations. While the materials in a triboelectric sensor cable are less expensive than those in either an electret or an inductive coupling type transducer cable, their performance is not as easily controlled. The transducer function of triboelectric cables can vary from cable to cable for no apparent reason, and their response voltages are not proportional to the amount of cable motion. Nevertheless, there are many triboelectric sensor cables in use today. A commercially available triboelectric sensor cable is the E-FLEX device produced by Stellar Systems Inc. of Santa Clara, Calif. U.S. Pat. No. 2,787,784 issued Apr. 2, 1957 and Canadian patent 1,160,300 issued Jan. 10, 1984 describe triboelectric transducer cables.
Known piezoelectric transducer cables use special plastic materials between two conductors in a coaxial cable construction. When flexed, such plastic materials generate a voltage which can be sensed at the termination of the able. The cost of the special plastic material is the major cost in the construction of the piezoelectric transducer cables. An example of a commercially available piezoelectric cable is the FOCUS device manufactured for Focus Ltd. by Chalice Electronics Ltd. of United Kingdom.
Fiber Optic transducer cables were introduced in the 1990's. Flexure of the fiber optic sensor cable alters the transmission of light along an optical fiber, and the effect of such alteration is detected at the end of the line. Fiber optic transducer cables tend to be relatively expensive due to the inherent cost of manufacturing the fiber optics therein. An example of a commercially available fiber optic cable sensor is FIBER SENSYS sold by Fiber SenSys, a Corning Affiliate in Beaverton, Oreg.
It should be noted that sometimes it is difficult to determine exactly how a transducer cable operates because triboelectric effects, electret effects and capacitive coupling all are sensed as a voltage at the end of the sensor cable, and hence all "look" somewhat alike electrically. To many users and some manufacturers, it does not matter how the sensor works as long as it reliably detects intruders. Many users treat the foregoing sensor cables as interchangeable, and simply purchase the lowest cost one.
For long outdoor protected area perimeters requiring multiple sensor units, the procurement and installation of suitable power and data networks are major factors in the total system cost. Due to the nature of perimeter security systems, these power and data transmission networks must be reliable and difficult to sever or spoof, i.e., deceive. The present invention includes a means of including the power and data transmission within the transducer cable. This eliminates the need to procure and install separate power and data lines around the perimeter of the protected area to service the multiple sensor "units". Since the transducer cable is tamperproof, the power and data service elements inside the cable are protected. In a closed perimeter system (one which encloses an entire protected area) the power and data can be supplied in both directions around the protected area perimeter to provide redundance.
My U.S. Pat. No. 4,562,428 describes the application of power and data over a two cable CW (continuous wave) leaky coaxial sensor cable. While the transmission of power and data over the transducer cable subsequently described herein has certain similarities to the system described in my U.S. Pat. No. 4,562,428, the present invention does not utilize leaky coaxial cables. In the present invention, electromagnetic waves are used to detect and locate disturbances inside the cable while U.S. Pat. No. 4,562,428 describes detecting disturbances in the outside air, between the two cables. In the present invention a radio frequency (RF) pulse is transmitted down the sensor cable, and the resulting received signals are processed to detect and precisely locate intruder-caused disturbances. In contrast, the system described in my U.S. Pat. No. 4,562,428 uses continuous wave (CW) transmissions with no capability of locating the intruder-caused disturbance along the lengths of the sensor cables.
My U.S. Pat. No. 4,091,367 describes a pulsed leaky coaxial cable sensor. While the RF pulsed disturbance locator system subsequently described herein also uses RF pulses to locate changes in characteristic impedance of coupled transmission lines, the present invention does not use leaky coaxial cables. Electromagnetic waves are used to detect and locate disturbance inside the sensor cable as opposed to disturbances in the air outside the cables.
A transmission line "presence sensor" is described in U.S. Pat. Nos. 3,750,125 and 3,801,976 by Ross et al. These patents describe a coupled strip line sensor in which the object being detected directly perturbs the electromagnetic coupling. This is similar to my leaky coaxial cable sensor described in U.S. Pat. No. 4,091,367 except that the lines are in close proximity to each other and are much shorter in length than in leaky coaxial cable sensors. In leaky coaxial cable sensors and the Ross et al device, the fields are not contained inside a cable, and the cable need not be physically disturbed by the target for the target to be detected.
U.S. Pat. No. 4,482,890 by Forbes et al describes a coupled fiber optic sensor for the detection and location of disturbances of a cable encompassing a multiplicity of fibers. U.S. Pat. No. 5,194,847 by Taylor et al describes a single fiber optic line with a directional coupler.
There clearly is an unmet need for a lower cost perimeter intrusion detection system capable of reliably detecting presence of an intruder, with much lower likelihood of false alarms from wind, hail, blowing debris and the like than previously has been achievable.